There is a new bug fix for the ethernet driver, just been merged to our github kernel tree on 4.14. This fixes a race condition that a couple of community member and PhilE of RPF(T) figured out. It may well explain a lot of the ethernet crashes reported by people on the 3b+, especially when using USB attached drives, or Samba, or things like Plex server.

It should be in rpi-update tomorrow, but in the meantime, you can build it yourself from here.